The single crystal of potassium niobate (KNbO.sub.3) is now attracting attention as a nonlinear optical material or an electro-optical material. Especially, its capability of converting semiconductor laser beams, being monochromatic rays of about 860 or 980 nm, to 1/2 higher harmonics is so high that the utilization thereof as a nonlinear optical material is attracting intense attention.
The single crystal of potassium niobate may be obtained by heating a powdery mixture of potassium carbonate and niobium oxide (Nb.sub.2 O.sub.5) at 1050.degree. C. or higher to thereby melt the same, implanting seed crystals in the melt and gradually cooling the melt so as to attain crystal growth, as described in T. Fukuda and Y. Uematu, J.J.A.P. 11 (1972) 163. During the cooling of the formed single crystal of potassium niobate, a structural phase transition from the cubic system to the tetragonal system occurs at about 420.degree. C., and another structural phase transition from the tetragonal system to the rhombic system occurs at about 210.degree. C. At room temperature, the single crystal of potassium niobate being of the rhombic system and being in the poly-domain state, is obtained.
In the use of the single crystal of potassium niobate as an optical element, e.g., as a wavelength conversion element or a light modulation element, conventionally, the single crystal of potassium niobate is subjected to the treatment for conversion to the single-domain state, i.e., poling treatment. Referring to FIG. 5, this poling treatment comprises respectively forming a positive electrode 2 and a negative electrode 3 on the c-plane 5 and the c-plane 6 of the single crystal of potassium niobate 1 by coating with a conductive paste containing conductive powder, such as silver or carbon powder, or by vacuum evaporation of gold or the like and subsequently applying an electric field of at least 1 kV/cm between the positive and negative electrodes 2, 3 sandwiching the single crystal of potassium niobate 1 at about 200.degree. C.
In this specification, the poly-domain state refers to a state in which a plurality of, or multiple adjacent domains having mutually different polarization directions are formed. The conversion of the poly-domain state to the single-domain state in which the polarization directions are uniformly arranged, is referred to as poling.
However, the above conventional poling treatment has a drawback in that it is difficult to pole the whole of the single crystal of potassium niobate, so that a poly-domain region remains, especially on the negative electrode side of the single crystal of potassium niobate.
Further, although the poly-domain region of the obtained single crystal of potassium niobate can slightly be decreased by increasing the electric field applied to the single crystal of potassium niobate or by extending the duration of the polling in the above conventional poling treatment, these unfavorably cause the single crystal of potassium niobate to suffer from quality deteriorations, such as coloring, resistivity decrease, and cracking.
Still further, in the above conventional poling treatment, application of a relatively high voltage while heating the single crystal causes the current to exhibit a gradual increase in spite of a constant voltage. Thus, there has been encountered the problem that the single crystal of potassium niobate is subjected to heat buildup, and to thereby suffers from cracking, or that a voltage drop is brought about with the result that no effective electric field can be applied to the single crystal of potassium niobate, e.g., practically only an electric field of about 1 kV/cm can be applied to the single crystal of potassium niobate when it is heated at 200.degree. C.
With respect to the poling of ferroelectric crystals, pages 830 and 831 of Crystal Engineering Handbook published by Kyoritsu Shuppan Co., Ltd. (Sep. 25, 1985) disclose a technique of poling a single crystal of lithium niobate by sandwiching the same between lithium niobate ceramics, and page 831, left column, lines 7 to 10 describe that, although there is a disorder of domain structure at the surface contacting the ceramic at multiple minute parts, the complete single-domain state is obtained at least 0.5 mm inside the surface. As apparent from the description, the technique described in this publication cannot convert the whole of the single Crystal to the single-domain state.
Japanese Patent Laid-open Publication No. 1(1989)-172299 discloses a technique of poling in which the single crystal of lithium tantalate is sandwiched, with the use of powdery niobic acid or a sintered body thereof as a medium, between a pair of platinum electrodes and voltage is applied between the electrodes.
The above two publications describe the poling of the single crystals of lithium niobate and lithium tantalate, but therein there is no description relating to the polling of the single crystal of potassium niobate.